The fact that the fuselage remained intact after the mistreatment of the crash landing is a testament to how modern aircraft design has advanced.
Modern aircraft design has advanced with respect to weight saving, older aiframes typically have more strength margins than newer ones. Except for those detials where regulations have changed (e.g. seat tracks)
Why in aviation, where although accidents are a lot more rare, are we still using just a single strap?
And why to we leave it to the passengers to tightem them (or not, which turns them from hip straps into deadly abdomen straps...)? Why don´t we have spring loaded inertia reel type of straps, which automatically are pulling tight?
But the answer to your question is much more simple: Because except for the window seats, there is no hard point to attach a third (or fourth) strap to!
If it would not be mandatory, a lot of people would probably not even wear their seat belt... Like many do in cruise and find themselves under the ceiling in heavy turbulence.